EGR Thermal Control Valve
Some models use a temperature sensitive control valve in vacuum line to EGR valve. Valve is closed below 61 °F, blocking vacuum to EGR valve and giving better cold driveability.
The thermal control valve is open above 100°F (1975-77) or 76°F (1978-79), engine temperature allowing EGR ported vacuum to be directed to EGR valve.
NOTE:
For proper hose routing on Thermal Vacuum Switches and Thermal Control valves, see GENERAL MOTORS VACUUM DIAGRAMS at end of in this section.